1. Field of the Invention
This invention relates to an arrester and a semiconductor circuit arrangement with a protection device where the arrester is used for protecting the semiconductor device.
2. Description of the Prior Art
One example of a conventional semiconductor circuit arrangement will be explained first. Recently a high voltage semiconductor conversion device including semiconductor valves, such as thyristor valves, which consist of a plurality of thyristors connected in series, has been used in a DC transmission system. In such thyristor valves protection of thyristors is very important problem. Shown in FIG. 1 is an example of a conventional system for protecting a thyristor valve when an overvoltage is applied thereto. In FIG. 1, reference character THV designates a thyristor valve to be protected, which is composed of a series circuit of thyristors SCR.sub.1 to SCR.sub.n, voltage dividers D.sub.1 to D.sub.n, an anode reactor AL, a gate pulse generator (not shown) and the like. Each of the voltage dividers D.sub.1 to D.sub.n includes a series connected circuit of a capacitor C and a resistor R. The thyristor valve THV is protected by an arrester Arrl from an overvoltage. The arrester Arrl is composed of a series connected gap G and nonlinear resistance element NR, in which an electrical discharge occurs at the gap G when a voltage appearing across the gap G exceeds a predetermined level, and any overvoltage energy is thus absorbed by the nonlinear resistance element NR so as to protect the thyristor valve THV.
In such a conventional device as shown in FIG. 1, a protection level against an overvoltage applied thereto is determined in accordance with the characteristics of the arrester Arrl. More specially, the protection level varies depending on various conditions such as the discharge condition at the gap G, the nonlinearity characteristic of the nonlinear resistance element NR and the like. According to the above factors, the number of thyristors SCR.sub.1 to SCR.sub.n which are to be connected in series in the thyristor valve THV, is determined.
FIG. 2 indicates the v-t characteristic curve, wherein v is a spark-over voltage of the arrester, t is a period of time during which the firing voltage v is applied to the arrester and V.sub.p is a protection level. In FIG. 2 the spark-over characteristics at the gap G exhibit a tendency that the spark-over voltage v increases in response to a voltage abruptly applied to the gap G. On the other hand, withstand voltages of thyristor elements SCR.sub.1 and SCR.sub.n decrease with respect to a forward voltage abruptly applied thereto as shown in FIG. 6, wherein v is a withstand voltage of a thyristor and t is a period of time during which the withstand voltage v is applied to the thyristor. Consequently, the protection level of the conventional arrester Arrl is generally set at a protection level V.sub.p which is obtained taking the abruptly changing voltage into consideration as shown in FIG. 2.
That is, as is apparent from FIG. 2, the protection level of an arrester with a discharge gap becomes higher. Therefore, the conventional protection system in which such an arrester is employed to protect a thyristor valve is disadvantageous in that the necessary number of thyristors increases, with the result that it is not economical.